Docker Kata 1: Basic Commands

Let’s get started with the first Docker kata. Each kata will be broken into steps. Below each command is a sample of the output that we should see when executing the command. Each step includes, after the commands, a breakdown of the commands and a summary of what they do.

Docker Kata 1 will introduce us to working with containers and images.

Step 1: Run the first container#

The command to run the container is given below.

The output will be something like this:

This first command runs a container from an image. A Docker image is a blueprint for a container. Containers are running instances of an image.

Note the first line of the output:

Unable to find image 'hello-world: latest' locally

Docker will first check the local image repository for the image indicated in the command. If the image is not found, it will check Docker Hub. Docker Hub is a public repository of images maintained by Docker Inc. The hello-world image is a test image stored on Docker Hub.

Image definitions include a start command. The start command runs a process inside the container when an image is run (we’ll work with that in later katas). The start command of the hello-world image echoes the text we see in the output window.

Step 2: List the containers#

The command to list the running containers is provided below.

After running this command, we’ll see output like the following:

The docker container ls command lists the containers. The first execution, however, returns an empty list. We ran the hello-world container in the first step, so why’s the list empty?

The list is empty because docker container ls only lists running containers. Containers run as long as their start process runs. If that process exits, the container exits. The start command for the hello-world image echoes the output from the first step and then exits immediately. The docker container ls command returns an empty list because no containers are running.

The command to list all the containers is given below.

The output of the command will be something like this:

The second command includes the -a parameter, which lists all the images, including those that have started and exited.

Step 3: List images#

The command to list all the images is given below.

When we run the command, the output will be something like this:

This command lists all the images in the local image repository. That will include all the images we’ve downloaded from Docker Hub and any we’ve created ourselves.

Step 4: Run a named container#

The command to name and run a container is given below.

The output will be something like this:

When running a container, we don’t have to specify a name. The Docker Engine will assign a random one. However, naming containers makes them easier to keep track of when running many at one time. The name assigned with the --name parameter will appear in the container list. That name can then be used to refer to the container in other commands, such as inspect.

The command to list all the containers is given below.

The output of the command above will be something like this:

It lists all the containers, including the ones running and exited.

The inspect command is given below.

The output will be something like this:

The inspect command works on several object types, including containers. When executed on a container object, inspect returns JSON-formatted data that describes the container.

We can also use a container ID to refer to a container when executing a command. The ID is a long string of characters. The first twelve of those characters are listed in the ID column:

devops@DevOpsKatas:~$ docker container ls -a
CONTAINER ID        IMAGE               COMMAND             CREATED       
c91a3a7c8134        hello-world         "/hello"            39 seconds ago
42eddbc93b81        hello-world         "/hello"            37 minutes ago

We can use the first three or more characters to refer to a container (and other objects with IDs):

devops@DevOpsKatas:~$ docker inspect 42e
[
 {
     "Id": "42eddbc93b81ca7c61c9abb385451d673e4e08293086966fa836cfd6e048a145",
     "Created": "2017-03-09T02:02:48.196355507Z",
     "Path": "/hello",
     "Args": [],
     ...

Step 5: Run a container in interactive mode#

The command to run a container in interactive mode is given below.

When we run the command, the output will be something like this:

Running a container interactively allows a user to connect to the container in a live fashion using a text console interface. This step runs a bash shell in the container, which is the same program we’re using when we enter commands in the terminal window. A container that’s run interactively behaves much like any other remote server. Developers and administrators can view files in the file system, run programs, and install components to the container. This mechanism is useful for debugging, testing, and defining new images.

If we look closely at the prompt in the bash shell console, we’ll see this:

root@e8fedb60e8e6:/#

The first part before the @ sign is the logged-on user. We connect as the root user to the container, which is the administrative superuser of a Unix OS. The second part after the @ sign is the computer name. The name of the container is its container ID:

devops@DevOpsKatas:~$ docker container ls -a
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS                      
e8fedb60e8e6        ubuntu              "bash"              2 days ago          Exited (130) 21 minutes ago

The command to list the files and folders is given below.

The output of the command above will be something like this:

The ls command lists the files in the container file system.

The command to list the bin directory is given below.

The output will be something like:

The ls /bin command lists the files in the bin folder.

The command to exit the container is given below.

The exit command exits the Bash shell running in the container.

Step 6: Remove all the containers and images#

The command to clear all the containers and images is given below.

When we run the command above, the output will be something like this:

This is a cleanup step that will be executed between some katas and steps to clear all the images and containers. These won’t be commonly run in a real production environment, given its potential to destroy work.

The command to list all the containers is given below.

When we run this command, the following output will be displayed:

The listing is empty because all the containers, running and exited, have been removed.

The command to remove images from the local computer is given below.

The output will be something like this:

This command removes all the images from the local computer.

Practice commands#

We’ve given a terminal at the end of the lesson and a table containing a list of commands discussed in this lesson. Try out these commands after running the terminal, and check out the results!